Police have about 70 terrorism investigations going on across the country, as ASIO signals concern that some children of foreign fighters may pose a jihadism risk if they return home.

Police and intelligence officials used a hearing of a terrorism legislation watchdog on Friday to make the case for measures such as bail-like control orders, and for declared no-go zones in Syria and Iraq to remain in place.

The Australian Federal Police's deputy commissioner for national security, Mike Phelan, told the Independent National Security Legislation Monitor, James Renwick, that it had "70-odd investigations" under way into terrorism cases.

The number is significant because it shows how many potential plots are sufficiently advanced for police to be gathering evidence with a view to launching prosecutions. It would include cases such as terrorism fundraising as well as possible attack plots, and also includes cases that are already before the courts.

ASIO has said before it has about 400 high-priority targets, but these are people of serious concern that the intelligence agency is watching, not necessarily specific plots that may lead to arrests and charges.

Mr Phelan said the last-resort option of seeking "control orders" was part of police planning in all 70 cases.

Control orders, issued by courts, impose bail-like conditions on terrorism suspects such as curfews and limits on who they speak to and how they use phones and computers.

While police prefer to arrest and charge people, control orders are available in extreme cases as a back-up when a person poses a risk but police cannot gather sufficient evidence, Mr Phelan said.

"Of the 70-odd investigations that we've got currently on foot within the [joint counter-terrorism teams], it is part of their planning for all of their investigations."

Photo: Andrew Meares

ASIO chief Duncan Lewis, above, said the agency expected an "ongoing trickle" of foreign fighters and their families returning home rather than a "major exodus" as the so-called Islamic State group lost territory on the battlefields of Syria and Iraq.

He said that of the roughly 200 Australians who had gone to the Middle East to fight with the group, about 100 remained.

Some 64 had been confirmed killed though the number might be as high as 72.

And there were 70 children the agency knew of that had either been taken to the conflict zone or born there.

While their return to Australia would present social challenges he warned that they could also pose a security risk.

"Not only will many of these individuals experience difficulties reintegrating into Australian society, but also many will be vulnerable to Islamist extremist ideology," he said.

He added: "Obviously there is quite an issue between whether somebody is a seventeen-and-a-half year old with an AK-47 or 17-month-old in wraps."

Mr Phelan said the AFP also strongly urged the continuation of "declared areas" offences, which make it a crime to enter designated zones controlled by terrorists. So far Syria's al-Raqqa region and Mosul in Iraq have been declared.

He said police faced challenges mounting other types of prosecutions given the well-known difficulties of obtaining admissible evidence from war zones or where there is no working relationship with authorities in that country as with Syria.